Replace parsing with existing library

[PragTob]

Citing @jasonrclark from over at shoes/shoes4#941

While we've extracted shoes-highlighter to its own gem, which is nice, it would be awesome to replace the Ruby syntax parsing with something someone else maintains which is likely to be more complete.

Potential resources to consider in doing this:

http://whitequark.org/blog/2012/10/02/parsing-ruby/
https://github.com/coatl/rubylexer
https://github.com/jneen/rouge
https://github.com/seattlerb/oedipus_lex

(Originally discussed on shoes/shoes4#257)

It was said that it might be easy to add a formatter for rouge for instance.